Cavity Growth Research Articles

Creep damage mechanism of 20Cr32Ni1Nb steel at high temperatures

In the petrochemical industry, the centrifugally cast austenitic 20Cr32Ni1Nb stainless steel used in steam reformer furnace's hot outlet manifold experiences creep failure and microstructural degradation, impacting its structural integrity. Understanding the causes of these issues is crucial for addressing the associated challenges. This study examines the creep deformation behaviour, damage mechanisms, and microstructural evolution of 20Cr32Ni1Nb steel under stresses of 22–50 MPa at temperatures of 1163 K (890 °C) and 1223 K (950 °C). Analysis of creep and creep strain rate curves revealed the presence of both primary and tertiary creep stages, with the power law relationship governing the stress-dependent minimum creep rate and rupture life. The variance of creep damage tolerance factor was also analysed, with results showing that in high-stress regimes, loss of section and necking along with cavity growth may initiate tertiary creep, while in low-stress regimes, microstructural degradation, specifically G-phase formation, may initiate tertiary creep. As the duration of creep exposure increases, microstructures were observed to degrade, with the growth and coarseness of NbC and M23C6 carbides. The structure of the space group for the Ni–Nb–Si enriched G-phase in 20Cr32Ni1Nb steel was found to be Fm3‾m, with a lattice parameter of about 1.14 nm.

The development of two multiaxial ductility factor predicting models based on creep cavity growth theory

The development of two multiaxial ductility factor predicting models based on creep cavity growth theory

Statistical Distribution of Critical Pitting Temperature of Stainless Steels

The range of Critical Pitting Temperature (CPT) for five different stainless steels is measured to determine whether it is wide or narrow. The impact of bulk solution composition and surface passivation on CPT was also measured in order to evaluate the relative importance of pit initiation and pit propagation on CPT. The results of the study revealed that CPT can be measured within a narrow range (±1.3 °C) regardless of the alloy and bulk solution composition. It is also shown that experimental error caused by crevice corrosion can lead to a wider range of CPT values. These results are explained using the Salinas-Bravo and Newman CPT model. It is shown that, for the conditions studied herein, CPT is not dependent on pit initiation, but rather, it is a transition phenomenon, i.e., CPT is determined by the transition from metastable to stable pit growth.

In situ cavity length control of GaN-based vertical-cavity surface-emitting lasers with in situ reflectivity spectra measurements

We developed an in situ cavity length control of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with an in situ reflectivity spectra measurement. Firstly, a temperature dependence of a center wavelength of a 40-pair AlInN/GaN DBR was investigated, resulting in a 23 nm redshift from RT to GaN growth temperature of 1100 °C. Secondly, a periodic oscillation of a reflectivity intensity during the GaN cavity growth on the DBR was clearly observed, providing precise cavity length information. Thirdly, based on both the results, we performed the in situ GaN cavity length control, showing an accuracy within a 0.5% error. We finally demonstrated the in situ cavity length control of an actual GaN-based VCSEL structure containing n-layers, a GaInN active region, and p-layers by selecting different resonance wavelengths along with the corresponding growth temperatures for the layers.

Creep damage in 9-12% Cr steel notched components: Microstructural evolution and stress state dependence

To understand the stress state dependent creep damage, following creep tests in 9–12% Cr steel notched components with different root radii, a study of associated time dependent microstructural evolution is presented in this work. Particular attention is paid to the dependence of creep damage evolution on the stress state. The average creep strain of notched component exhibits a typical three-stage characteristic (transient, steady and tertiary stages). During the transient stage, remarkable re-distribution in stress state (equivalent stress and stress triaxiality) is found. The rapid relaxation of equivalent stress near the notch root, associated with the initial coarsening of substructure would delay the nucleation or growth of cavity. By contrast, the time-dependent increase of equivalent stress combined with the relatively high stress triaxiality seems to be responsible for the higher cavity density at the center of notched component. The precipitate coarsening is heterogeneous and promoted by the high level of equivalent stress or stress triaxiality. During the steady stage, the creep damage behavior is related to the magnitude of stress state. Higher cavity density and greater reduction in hardness are mainly found in the region where both the equivalent stress and stress triaxiality are high. As a whole, both the redistribution process of stress states and their levels exhibit significant influence on microstructural evolution and creep damage behavior of notched components at elevated temperatures.

Factors influencing microbial growth in the human oral cavity

The human oral cavity is a significant contact point for diverse microbes. It allows frequent entry and exit of microbes through inhalation, exhalation, drinks, spitting, and consuming foods. The environmental conditions inside the cavity influence the selection, growth, and development of microbial communities. The conditions, in turn, are probably because of complex interaction and activities of various molecules of host and residential microbes. Further, oral ecology is influenced by the food habit of individuals and the community. The community of microorganisms may shift gradually or spontaneously depending on the physicochemical environment in the cavity. The presence of a group of organisms along with the food habit jointly may indicate the probable microbes succeeding. Understanding the relation complexity among the physiochemical nature of the oral cavity, the resident microbes, and individuals’ food habits may decipher the microbes to be attracted for succession. This review is for understanding the factors that influence the occurrence and refusal of oral microorganisms.

Water entry of cups and disks

It is known that the water entry of a body with a recessed, cupped nose can suppress the splash and air cavity typically observed for solid body entry (Mathai, Govardhan & Arakeri, Appl. Phys. Lett., vol. 106, 2015, 064101). However, the interplay between the captive gas in the cup, the cavity and the splash is quite subtle and has not been thoroughly explored. Here we study the cavity and splash dynamics associated with the vertical water entry of cups and find a variety of regimes over a range of Weber numbers ( $We_D$ ) and dimensionless cup depths. Our parameter space spans a transition between slow-developing cavities with long closure times (low $We_D$ ) to fast-sealing cavities (high $We_D$ ). An important dynamic event is the evacuation of trapped gas from within the cup, which drives the ensuing cavity and splash behaviour. Through modelling, we predict the conditions for which the evacuating gas inflates a cavity that opens to the atmosphere versus inflating a submerged cavity that suppresses air entrainment from above the surface. We also compare our cup water entry findings to the impact phenomena observed for flat disks, which entrap gas on the front surface similar to cups. In doing so, we reveal the sensitivity of disk splash and cavity behaviour to impact angle, and show that disks share a common regime with cups, in which a thin splash quickly seals on the body. We deduce the mechanisms by which increasing cup depth delays the cavity seal time in this regime. These findings reveal that cups may in fact promote or suppress cavity growth, depending on the cup depth and impact conditions.

Effect of Heat Treatment on the Cavitation Erosion Behavior of Nanocrystalline Surface Layer of 304 Stainless Steel

In this study, a nanocrystalline layer composed primarily of martensite phase was prepared on the surface of 304 stainless steel. Furthermore, the martensite phase content in the nanocrystalline layer was adjusted by heat treatment at 500 °C and 550 °C, respectively, and the cavitation erosion resistance of the nanocrystalline layer before and after heat treatment was investigated. The results showed that the nanocrystalline layer before and after heat treatment exhibited excellent erosion resistance, with cumulative mass loss of approximately 1/7, 1/5, and 1/3 that of the traditional 304 stainless steel, respectively. The nanocrystalline layer could significantly inhibit the growth of cavitation pits due to the high density of grain boundaries. However, due to the decrease in hardness of the nanocrystalline layer after heat treatment, the propagation speed of cavitation cracks was accelerated, and the cavitation erosion performance of the nanocrystalline layer showed a downward trend.

Eulerian–Lagrangian modeling of phase transition for application to cavitation-driven chemical processes

Hydrodynamic cavitation is a promising technology for several applications, like disinfection, sludge treatment, biodiesel production, degradation of organic emerging pollutants as pharmaceutical, and dye degradation. Due to local saturation conditions, cavitating liquid exhibits generation, growth, and subsequent collapse of vapor-filled cavities. The cavities' collapse brings very high pressure and temperature; this last aspect is essential in some chemical processes because it induces the decomposition of water molecules into species with a high oxidation potential, which can react with organic substances. Properly exploiting this process requires a highly accurate prediction of pressure peak values. To this purpose, we implemented a multi-phase Eulerian–Lagrangian code to solve the fluid-dynamic problem, coupled with the Rayleigh–Plesset equation, to capture the evolution of bubbles with the required accuracy. The algorithm was validated against experimental data acquired with optical techniques for different cavitation-shedding mechanisms. Then, we used the developed tool to investigate the decoloration of organic substances from a cavitation Venturi tube operating at different pressure. We compared the obtained results with the experimental observation to assess the reliability of the developed code as a predictive tool for cavitation and the possibility of using the code itself to assess scale-up criteria for possible industrial applications.

REMINERALISING AGENTS IN DENTISTRY-A REVIEW

The main component of the human dentition is hydroxyapatite, which is a highly mineralized tissue. The weight of all oral diseases worldwide is exacerbated by dental caries. Caries is not a one-way process that only involves demineralization; rather, it is a cyclical process with stages of both demineralization and remineralization. The intricate interplay between saliva, diet, and bacteria causes demineralisation leading to cavitation. With the aid of the right tools and even non-fluoride remineralization methods,initial non-cavitated lesions can be repaired. Using a range of topical remineralizing treatments, the formation of fluorapatite in enamel and white spot lesions is inhibited and remineralized. The minerals, including ions and hydroxyapatite crystal lattices, are replenished naturally through a process known as rremineralization. Putting a strong emphasis on the remineralization concept is the best strategy for managing caries. New mineralization agents have been developed to preserve the enamel's structure and prevent the growth of carious cavities. The goal of modern dentistry is to halt the progression of disease and enhance appearance, tensile strength, and function by remineralizing non-cavitated caries lesions

Grain boundary softening from stress assisted helium cavity coalescence in ultrafine-grained tungsten

The formation of helium cavities in coarse-grained materials produces hardening proportional to the number density and size of the cavities and due to the interaction of dislocations with intragranular helium defects. In nanostructured metals containing a high density of interfacial sinks, preferential cavity formation in the grain boundaries instead produces softening that is often attributed to enhanced interfacial plasticity. Employing two grades of ultrafine-grained tungsten, we explore this effect using targeted implantation studies to map cavity evolution as a function of the irradiation conditions and quantify its impact on the mechanical response through nanoindentation. Softening is reported at implantation temperatures above the threshold for preferential grain boundary cavity formation but at a sufficiently low fluence prior to the growth of intragranular cavities. Collective changes in the mean cavity size, density, and morphology beneath a residual impression on an implanted surface indicate that cavity coalescence accompanied the reduction in hardness. Complementary atomistic simulations demonstrate that, in tungsten grain structures exhibiting softening, grain boundary bubble coalescence is driven by stress concentrations that further act to localize strain in the grain boundaries through cooperative deformation processes involving local atomic shuffling and sliding, dislocation emission, and even the nucleation of unstable twinning events.

Optimization of electroless Ni B-nanodiamond coating corrosion resistance and understanding the nanodiamonds role on pitting corrosion behavior using shot noise theory and molecular dynamic simulation

This study aimed to optimize the corrosion resistance of the electroless NiB-nanodiamond coating using a response surface method (RSM) by changing the electroless bath parameters and understand the role of nanodiamonds on pitting corrosion behavior using linear polarization, and electrochemical noise measurements as well as molecular dynamics simulation. The structure of optimized coating and nanodiamonds (ND) were characterized using FTIR, XPS, GDOES, SEM, and TEM. The stability of nanodiamonds was improved through acid treatment in HNO3 and H2SO4 (volumetric 1:3) solution. The quadratic model for predicting the polarization resistance of coatings based on the designed experiments was proposed with an accuracy of 92.85 %. Calculation of the pit initiation rate based on Weibull probability plots and probability of non-exceedance of pit diameter from shot noise theory reveals that the pit nuclei form at almost the same rate for NiB and NiB-nanodiamond coating, however, in NiB-nanodiamond coating the growth of pits is stopped. The molecular dynamic simulation showed that chloride ions forms weak chemical bonding on the ND surface than nickel due to the lower adsorption energy between the nanodiamonds surface and chloride ions which block localized corrosion attack.

Study of imitative micro pit morphology evaluation

This study examined the imitative micro pit formation of pitting corrosion on 304 stainless steel under electrochemical control in a 3.0% NaCl solution. Results demonstrated that the micro pits changed from a conical to a cap shape during growth, indicating that their evolution in shape and growth did not follow the same morphology over time. Additionally, the piecewise polynomial method was employed to compare real-time micro pit growth morphology evolution with current density, which was consistent with cross-sectional images of actual micro pits, enabling the separation of the pitting corrosion stages. Furthermore, 3D images obtained from SRXTM were similar to cross-sectional images of real micro pits, further validating the technique’s usefulness in studying micro pits.

Corrosion test and three-dimensional cellular machine simulation of Q235 steel by ion diffusion in different soil environments

In the actual soil environment, due to the unpredictable characteristics of metal pitting corrosion. Therefore, in this study, soil composed of different concentrations of corrosion medium was used for the mental corrosion test, and the experimental results of single ion and multiple ions were compared. At the same time, the corrosion behavior of metals is introduced in combination with the mathematical model of unsteady diffusion. In addition, the cellular automata model verification method is used to study the comparison and verification of the diffusion growth model of the model corrosion pit in different iteration steps under the discrete mathematical model. The results show that the growth and diffusion trend of corrosion pits controlled by the concentration of SO42-, HCO3–, and Cl- is consistent basically with the law reflected by cellular automata.

The divergent angle of cavitational devices influences melanoidin degradation. A new look through experimental and computational analysis

In the present work, the influence of the divergent section angle of hydrodynamic cavitation (HC) devices on melanoidin degradation was investigated. The HC reactor was operated alone and in combination with an oxidant at four concentrations of H2O2: 0.9, 2.8, 4.7, 6.6 g L−1. Five Venturi devices (with divergent angles of 4°, 8°, 11°, 14°, and 90°) were made and analyzed at five operating pressures (150, 300, 440, 600, 760 kPa) for a period of 30 min. ANOVA and Fisher's LSD method were used to define the best angle and the best operating pressure for melanoidin degradation. The highest efficiency of melanoidin degradation (14.32 %) was obtained with an angle of 11° and a pressure of 600 kPa. Computational Fluid Dynamics (CFD) simulations were performed to study the influence of the divergent angle on the flow parameters. They showed that at smaller angles (4° and 8°) there was an excess growth of cavities, increasing the chance of coalescence of cavities and reducing the intensity of collapse. On the other hand, at a very high angle (90°), the cavities collapsed before growing, releasing less energy at the time of implosion. The degradation efficiency increased to 60.84 % when 2.8 g L−1 of H2O2 was applied to the HC reactor, at pH 2.5. Higher concentrations of H2O2 reduced the degradation efficiency, indicating a “scavenging” effect in combined systems (HC + H2O2) due to the generation of unreactive radicals. The present study demonstrated that the combination of HC + H2O2 is a promising alternative for melanoidin degradation, with the divergent angle of the device being an important parameter of this treatment process.

Prolonged creep lifetime of ferritic self-healing steels achieved by offline healing treatment

For the ferritic self-healing (FSH) steel, it remains challenging to solve the low healing efficiency problem, due to the mismatch between the formation kinetics of creep cavities and the precipitation kinetics of healing agents. In this research, an offline healing treatment has been proposed to stimulate the formation of the Laves phase precipitates as self-healing agents to retard creep cavity growth at elevated temperatures. The corresponding treatment parameters were determined by the precipitation behaviors of Laves phases on the external free surfaces and internal cavity surfaces of FSH steels. Results have shown that the creep lifetime of the offline healing treated samples can be extended more than 30% compared to that of non-healed sample, while the creep rates can be reduced by one order of magnitude. The formation kinetics of Laves phases and creep cavities have been quantitatively analyzed. It was found that after the offline healing treatments, the average cavity sizes of the samples decreased significantly, while the growth rates of creep cavities rather than the nucleation kinetics are compensated by the healing phenomena of Laves phase formation. The relationship between the extended lifetime and filling ratios of creep cavities by the healing agents was established as well. The demonstrated potential of offline healing treatments provides a new strategy to cultivate the self-healing capabilities of heat resistant steels.

Comparison of Metastable Pitting Behavior of Austenitic and Duplex Stainless Steels under U‐Bending Deformation

The metastable pitting behavior of austenitic stainless steel (304 SS) and duplex stainless steel (2205 DSS) under U‐bending deformation is comparatively investigated. This study highlights the effects of differences in inclusions and matrix between 304 SS and 2205 DSS with regard to the metastable pitting behavior under tensile and compressive deformation. The results show that the pit initiation frequency of 304 SS increases in the order: tensile < as‐received < compressive; in contrast, the initiation frequency of 2205 DSS increases in the order: as‐received < tensile < compressive. In addition, tensile and compressive deformation change the preferential initiation sites of metastable pitting in 2205 DSS. It is proposed that the types of transient signals can be comprehensively and objectively distinguished by the two parameters (t g/t rep) and [(I p/Q g):(I p/Q rep)]. The correlation relationship between transient signals and pitting morphologies is clarified by cluster analysis. Both tensile and compressive deformations promote the growth of a few metastable pits, thus increasing the transition probability from metastability to stability. Overall, both tensile and compressive deformations are detrimental to the pitting corrosion resistance of the experimental steels.

Creep-fatigue damage mechanisms and life prediction based on crystal plasticity combined with grain boundary cavity model in a nickel-based superalloy at 650°C

In this study, a dual-scale numerical procedure is developed to reveal the creep-fatigue damage mechanisms and estimate the crack initiation life for notched structures made of Inconel 718 superalloy at 650 °C. The macro-scale simulation solves the creep-fatigue deformation behavior with viscoplastic constitutive models, and the local deformation histories are supplied to the micro-scale simulation as boundary conditions. In the micro-scale simulation, the local damage evolutions are solved based on crystal plasticity combined with grain boundary cavity model. The creep damage is calculated by a special formulation in the form of cavity nucleation, growth and coalescence. The fatigue damage is represented by accumulated energy dissipation originated from crystal plasticity finite element simulation. Experimentally, the creep-fatigue tests of notched structures are carried out for Inconel 718 superalloy at 650 °C to validate the feasibility and robustness of the proposed numerical procedure. Moreover, the crack initiation behavior, including transgranular cracks under fatigue loading and intergranular cracks under creep-fatigue loading, is explained through different types of microstructure observations together with a dual-scale numerical procedure. In detail, the crack initiation sites transferred from the grain interior at notched surface to the grain boundaries at notched subsurface with an increase in hold times can be well predicted by the proposed numerical procedure. In addition, the simulated life based on the developed life prediction approach agrees well with the experimental data within an error band of ±2. Parametric studies show that the creep damage is more sensitive to grain boundary diffusion than to the external conditions of strain level and hold time.

Effect of grain size on superplastic deformation behavior of Zn-0.033 Mg alloy

Zn alloys reveal excellent biological degradability and compatibility, suitable mechanical properties, and unique anti-atherosclerotic properties, which make them suited for miniaturized medical implants. Superplastic forming is a promising method to fabricate high-performance microparts with difficult-to-deform materials. However, the effect of grain size on superplasticity in Zn alloys and the underlying mechanisms still need to be determined. In this research, Zn-0.033 Mg (in wt.%) alloy with different grain sizes from 1.53 to 36.17 μm was obtained by equal-channel angular pressing and heat treatment, and its mechanical performance was evaluated by uniaxial tensile tests in the temperature range of 453–533 K and the strain rate range of 0.001–0.01 s−1. Experimental results indicated that the flow stress is significantly reduced with increasing deformation temperature, and the elongation is effectively improved with the reduction in strain rate. Furthermore, the Zn-0.033 Mg alloy with a grain size of 1.53 μm exhibited a superplastic deformation with an elongation of 376 ± 79% at 453 K and 0.001 s−1. Two distinguishing deformation stages were observed during the superplastic deformation. In the early stage, the ∼85 ± 5°/ <1‾1‾20> twins significantly emerged, leading to subsequent dynamic recrystallization by high-angle grain boundary migration. In the steady-state stage, the non-basal dislocation slip was significantly activated via changing the grain orientation from hard to soft orientation with the aid of continuous dynamic recrystallization. The fracture mechanisms were further discussed based on the fracture surface analysis, indicating that the ultrafine-grained structures can accommodate the nucleation and growth of cavities by grain boundary sliding, enabling low-temperature superplasticity. These findings can grant a new insight and understanding of the superplastic mechanism of Zn alloys and shed light on the manufacture of high-performance Zn alloys for medical implants.

Shear-band cavitation determines the shape of the stress-strain curve of metallic glasses

Metallic glasses are known to have a remarkably robust yield strength, admitting Weibull moduli as high as for crystalline engineering alloys. However, their postyielding behavior is strongly varying, with large scatter in both flow stress levels and strains at failure. Using x-ray tomography, we reveal how a strain-dependent internal evolution of shear-band cavities underlies this unpredictable postyielding response. We demonstrate how macroscopic strain softening coincides with the first detection of internal shear-band cavitation. Cavity growth during plastic flow is found to follow a power law, which yields a fractal dimension and a roughness exponent in excellent agreement with self-similar surface properties obtained after fracture. These findings demonstrate how internal microcracking coexists with shear-band plasticity along the plastic part of a stress-strain curve, rationalizing the large variability of plastic flow behavior seen for metallic glasses.